A wide variety of applications use photodiodes to convert optical signals into electrical signals. In practice, however, photodiodes typically are custom made for specific applications. For example, many optical telecommunications applications transmit light having wavelengths of between 850 to 1600 nanometers. The materials and properties of photodiodes used in those applications thus are selected to ensure optimal operation with light having those wavelengths.
To that end, many optical telecommunications applications use germanium-based photodiodes (i.e., photodiodes having primarily germanium and some other element, such as a group four element), which operate satisfactorily with the anticipated wavelengths. This type of photodiode (e.g., a germanium-based PIN photodiode), however, still has a number of operational problems.
Specifically, as background, a germanium PIN photodiode has an n-type doped region, a p-type doped region, and an intrinsic region (i.e. slightly doped or not doped) between the two doped regions. A sufficient potential difference is applied between the doped regions, which produces a current when illuminated by light of the intended signal wavelength. The strength of the resultant photocurrent is based upon the amount of incoming light signal absorbed by the intrinsic region. Optically generated electron-hole pairs in heavily doped germanium, however, typically have a short lifetime as minority carriers and rapidly recombine. Consequently, such pairs do not reach the proper photodiode electrode to contribute to the output current. Accordingly, to maximize the amount of light absorbed by the intrinsic region, the thickness of the doped regions is minimized relative to that of the intrinsic region.
Reducing the thickness of the doped regions (especially the doped region first receiving the incoming light—often referred to as the “topside electrode”), however, reduces their conductivity. Consequently, the photodiode cannot easily transmit the current it produces. In other words, current produced by an incoming light signal still effectively is attenuated because the doped regions do not have enough conductivity to transmit the current to an attached lead or other current transmission device.
The art has responded to this problem in a number of ways. One solution increases conductivity by coupling metal electrodes to the doped region that receives the light to be converted. Undesirably, however, the metal electrodes block some of the incoming light signal, still attenuating the resultant electrical signal. Moreover, because of thermal and material mismatches, germanium-based photodiodes present additional challenges when integrated with silicon integrated circuits (e.g., CMOS or bipolar technologies).